1. Field of the Invention
The present invention pertains to a temperature indicator for refrigerated products, using the solidification and melting of liquid bodies at room temperature.
More specifically, the present invention relates to the verification that the temperature is maintained either between two limits or below or above a critical temperature, and it is especially designed to verify that fresh or frozen products have not spoiled unbeknownst to the users, as the result of a power failure or malfunction of the cooling circuits of the refrigerated transport or preservation enclosure. The present invention mainly relates to food products, but may also be used in any situation where a storage temperature must be maintained, for example, for pharmaceutical products, or unstable products such as paints, adhesives, etc.
2. Discussion of Background Information
Frozen or deep-frozen food products must be preserved at a temperature remaining permanently below a determined temperature. For fresh products, transport and preservation must take place at a temperature comprised between zero and seven degrees or less, and must not rise above or below these values. However, a substantial temperature variation may occur accidentally, without warning to the product distributor or consumer in the case where the temperature comes back to a correct value during verification. The problem is all the more difficult to resolve because the products are often displayed in glass cases and are accessible to the buyer who may touch them, remove them from the refrigerated enclosure, and return them after having examined them.
Similarly, at several instances during their transport, the products pass outside of the cold preservation or transport enclosures, at temperatures that are difficult to control.
These considerations clearly indicate that a temperature monitor limited to cold transport, storage or display enclosures is inadequate. In order for this monitor to be efficient, the crossing of allowable temperature limits inside any individual packaging must appear by means of an easily visible sign, which remains after the temperature is back within the authorized limits. To date, devices enabling detection of such a temperature variation are either costly electronic alarm systems intended only for substantial installations, or monitors affixed to the product packaging that change color when the temperature rises, and are influenced especially by the temperature of the enclosure.
At the present time, these indicators are not widely distributed, and to our knowledge, there is no efficient and inexpensive means enabling detection of a dangerous temperature variation, especially between two values.
A certain number of patents, and specifically FR 90 08 518 and FR 90 10 498 (published as French Patent Document No. 2,665,957), by the same inventor, suggest using monitors constituted of capsules containing an aqueous solution whose freezing or thawing causes a visible, irreversible phenomenon so as to determine that a critical temperature has been crossed, upwards or downwards. These devices, although they resolve the problem of production cost, suffer from a serious disadvantage prohibiting their development in practice. In particular they must be conditioned or "stimulated" before or after freezing the product to be preserved, which not only requires handling that is difficult to imagine for large production quantities, but limits monitoring reliability, especially in the case where this stimulation is reversible.